As seen in the diagram, impure titanium, zirconium,
hafnium,
vanadium,
thorium or
protactinium is heated in an evacuated vessel with a
halogen at 50–250 °C. The patent specifically involved the intermediacy of
TiI4 and
ZrI4, which were volatilized (leaving impurities as solid). At atmospheric pressure
TiI4 melts at 150 °C and boils at 377 °C, while
ZrI4 melts at 499 °C and boils at 600 °C. The boiling points are lower at reduced pressure. The gaseous metal
tetraiodide is decomposed on a white hot
tungsten filament (1400 °C). As more metal is deposited the filament conducts better and thus a greater electric current is required to maintain the temperature of the filament. The process can be performed in the span of several hours or several weeks, depending on the particular setup. Generally, the crystal bar process can be performed using any number of metals using whichever halogen or combination of halogens is most appropriate for that sort of transport mechanism, based on the reactivities involved. The only metals it has been used to purify on an industrial scale are
titanium,
zirconium and
hafnium, and in fact it is still in use today on a much smaller scale for special purity needs. ==References==